This invention relates to a fixing unit in a copying machine, and more particularly to a device for driving a pressing roller in the fixing unit.
In general, in a fixing unit of a copying machine, and especially in a fixing unit using a fixing roller and a pressing roller, the fixing roller and the pressing roller are brought into contact with each other so that toner transferred onto an image supporting sheet is made molten and is fixed by the heat of a heater incorporated in the fixing roller and the nipping pressure of these two rollers. If the fixing roller and the pressing roller are maintained in contact with each other as described above, then an elastic part which forms the surface of the pressing roller may be deformed by being heated. Furthermore, toner offset onto the fixing roller may then be offset to the pressing roller, which may spoil the back side of the next sheet. In order to overcome these difficulties, a sensor is provided near the fixing roller so as to detect when sheets go into and leave the fixing roller. In response to a detection signal from the sensor, the pressing roller is moved into and out of engagement with the fixing roller.
The operation of moving the pressing roller into and out of engagement with the fixing roller is carried out by a pressing roller drive device. A conventional pressing roller drive device comprises an arm coupled to the pressing roller; an eccentric cam for moving the arm up and down; a spring clutch for selectively transmitting the drive force of a motor to the cam shaft of the eccentric cam; and a solenoid for periodically energizing the clutch. The spring clutch has two pawls, one of which is engaged with the end of a pivotally operating arm, the other end of which is coupled to the solenoid. When the pivotally operating arm is in engagement with the pawl, the pressing roller is in contact with the fixing roller or is set apart from the latter. In this case, the drive force of the motor is not transmitted to the cam shaft, and therefore the cam is at rest. Upon energization of the solenoid the arm is disengaged from the pawl, as a result of which the spring clutch engages the cam shaft to transmit the motor's drive force, to thereby turn the cam shaft. Therefore, the cam is turned to move the pressing roller vertically. Since the solenoid is energized only momentarily, after the spring clutch turns through 180.degree., the arm is engaged with the next pawl. That is, whenever the cam makes a half revolution, the pressing roller is moved into and out of engagement with the fixing roller.
However, in the above-described pressing roller drive device, in separating the pressing roller from the fixing roller the fixing roller reacts on the pressing roller so that the speed of rotation of the cam shaft becomes higher than that of the motor and the arm cannot sufficiently follow the cam surface of the eccentric cam, with the result that the arm drops onto the cam surface abruptly, thus generating a large impact noise.